1. Field of the Invention
The present invention relates generally to a twin ignition plug control system for an internal combustion engine and more specifically to an ignition control system which can switch two-point ignition to one-point ignition or vice versa according to engine load in a fuel-injection type internal combustion engine having two ignition plugs for each engine cylinder.
2. Description of the Prior Art
As is well known, there exists some twin ignition plug control system for a fuel-injection type internal combustion engine which can switch from two-point ignition to one-point ignition or vice versa for each engine cylinder according to the magnitude of engine load.
An exhaust gas recirculation system is often adopted for an internal combustion engine, in order to reduce NOx exhausted from the engine, by recirculating part of exhaust gas from the intake port to the exhaust port and thus lowering the combustion temperature. This recirculation system, however, is usually disabled when the engine is running at a low speed or when engine coolant temperature is low, that is, when the engine is running under relatively heavy load or is being idled, in order to obtain a reliable and stable engine operation.
In order to obtain more reliable ignition performance even when exhaust gas is being recirculated, two-point ignition method is adopted in which an ignition plug is disposed on the intake port side and on the exhaust port side, respectively, for each engine cylinder for increasing the combustion speed in the mixture including the recirculated exhaust gas.
In the two-point ignition systems, a relatively-intense combustion noise is produced when the two plugs are ignited simultaneously while the engine is running under heavy load. Therefore, when the engine load exceeds a predetermined level, the two-point ignition is switched to a one-point ignition to reduce combustion speed and thus reduce combustion noise.
To detect heavy engine loads, prior-art two-point ignition systems usually comprises a vacuum switch disposed within the intake manifold, which is opened (or closed) when the absolute pressure within the intake manifold exceeds a predetermined pressure (approximately -80 mm Hg in gage pressure), that is, when engine load becomes heavy. In response to this switch signal indicative of heavy load, the ignition system is switched from two-point to one-point ignition.
In the prior-art two-point ignition system, however, while the engine is being started or warmed-up, since the pressure in the intake manifold rises as high as atmospheric pressure and thus the vacuum switch is opened as if engine load were heavy, the system is switched to one point ignition, in spite of the fact that two-point ignition is preferable in order to improve engine starting or idling performance.
To overcome this problem, the prior-art two-point ignition system usually comprises a clutch switch linked with the clutch pedal or the gear shift lever of the transmission mechanism which close when the engine is being started or warmed-up, or, in the case of an automatic transmission vehicle, a neutral switch is closed when the gear shift lever is set to the neutral or park positions in order to switch the systems to two-point ignition.
In summary, in the prior-art two-point ignition system, since heavy load is detected with a vacuum switch, in dependence upon change in absolute pressure within the engine intake manifold, in order to switch the control system from two-point to one-point ignition, a clutch switch or a neutral switch (or inhibit switch) is additionally required for preventing the system from being switched from two-point to one-point ignition while the engine is being warmed-up, thus resulting in a more complicated system configuration and a higher manufacturing cost.
Furthermore, there has been proposed another prior-art two-point ignition control system, by which two-point ignition is switched to one-point ignition when engine load is determined to be heavy in dependence upon the pulse width of a fuel-injection valve actuating signal. In this system, the pulse width is calculated on the basis of the amount of air supplied into the engine (detected by an air-flow meter). In such a prior-art two-point ignition control system, however, although there is no vacuum switch as described in the first prior-art system, there exists another problem in that two-point ignition is switched to one-point ignition whenever the pulse width of the fuel-injection valve actuating signal increases, for instance, due to engine idling, in spite of the fact that two-point ignition is necessary to improve ignition performance when the engine is being warmed-up.